In Japanese Published Patent Application No. 2003-133555, a semiconductor device in which a peripheral electrode is formed along a circumference of an upper surface of a semiconductor substrate is disclosed. The peripheral electrode is electrically connected with a semiconductor layer. Because the peripheral electrode is formed, when the semiconductor device is not conducting, a depleted layer extending from a semiconductor element region (the semiconductor element region is a region in which a semiconductor element is formed) is terminated in the semiconductor layer under the peripheral electrode. Therefore, the depleted layer is prevented from reaching the circumference of the semiconductor substrate. As a result, electric field is prevented from being applied to the circumference of the semiconductor substrate. On the other hand, if the peripheral electrode is formed, the depleted layer ends in the semiconductor layer under the peripheral electrode and the electric field is concentrated into that semiconductor layer. Therefore, an electric strength of the semiconductor device can not be increased so much merely by the peripheral electrode.
As a technique for solving the above problem, a semiconductor device having one or more intermediate electrodes, which are formed on the surface of the semiconductor substrate between the semiconductor element region and the peripheral electrode, is known (for example, Japanese Published Patent Application No. 2005-209983). The intermediate electrode is insulated from the semiconductor element region and the peripheral electrode. Furthermore, the intermediate electrode is formed on an insulating film and insulated from the semiconductor layer. That is, the intermediate electrode is in a floating state. Therefore, the intermediate electrode has an electric potential between an electric potential of the semiconductor element region and an electric potential of the peripheral electrode. If the intermediate electrode is formed, an electric potential of the semiconductor layer below the intermediate electrode corresponds to the electric potential of the intermediate electrode. Therefore, extension of the depleted layer from the semiconductor element region toward a side of the circumference is suppressed at the semiconductor layer below the intermediate electrode. That is, both of the intermediate electrode and the peripheral electrode suppress the extension of the depleted layer. As just described, in this semiconductor device, because the extension of the depleted layer is suppressed at the two points, the concentration of the electric field does not easy occur compared with the semiconductor device not having the intermediate electrode. By forming the intermediate electrode, the electric strength of the semiconductor device can be improved.